The Pentagon will resume flights of its F-35 Joint Strike Fighter jets, which were grounded a week ago after a crack was discovered in an engine of a test plane,

Matthew Bates, a spokesman for engine manufacturer Pratt & Whitney, said the Pentagon's F-35 program office had decided to lift a temporary suspension of flight operations after it conducted extensive tests on the affected engine part.

The tests showed a crack in a turbine blade stemmed from the "unique operating environment" in flight tests rather than a design flaw, he said.

Mr Bates said Pratt had been working around the clock with Pentagon officials to determine the cause of the crack in the engine blade.

"The team has determined that root cause is sufficiently understood for the F-35 to safely resume flight," he said.

He said the engine in question had operated at high temperatures more than four times longer than a typical F-35 flight, which led to a separation of the "grain boundary" of this particular blade.

Pratt, a unit of United Technologies Corporation, supplies the engine for the single-engine, single-seat fighter plane, which is built by Lockheed Martin Corp.

The Pentagon announced the grounding of all F-35 warplanes almost a week ago after an inspection revealed a crack on a turbine blade in the jet engine of an F-35 being tested at Edwards Air Force Base in California.

It was the second engine-related grounding in two months of the $US396 billion F-35 Joint Strike Fighter, the Pentagon's largest weapons program.

The Marines Corps version of the plane was grounded for nearly a month starting in mid-January because of a faulty hose in the engine.